1. Field Of The Invention
This invention pertains to a method for minimizing the formation of surface carbides on steels during annealing. More particularly, it pertains to a method for minimizing surface carbide formation during box annealing of drawing quality specially-killed steels.
2. Description Of Related Art
Aluminum-killed steels, both continuous cast and ingot cast, are often used in applications requiring superior drawing characteristics, such as sheets and coated sheet products. To impart the desired texture and drawing characteristics to these steels, coils are box annealed at temperatures above 1250.degree. F. The product of the annealing process is a drawing quality, specially-killed (DQSK) steel.
While the box annealing process imparts the desired formability characteristics, it is not without drawbacks. In commercial charges, coil temperatures often exceed the ferrite to austentite transition temperature (A.sub.1 temperature), approximately 1340.degree. F. to 1350.degree. F. for low carbon steels. At temperatures above the A.sub.1 temperature, aluminum-killed steels are susceptible to the formation of surface carbides. These carbides, which consist mainly of cementite (Fe.sub.3 C.). are undesirable. W.H. McFarland has characterized their formation during batch annealing as "[o]ne of the more mystifying phenomena in the production of cold rolled steel sheets." W. H. McFarland, Surface Carbides In Batch Annealed Cold Rolled Steel, 1988 Mechanical Working and Steel Processing Proceedings 155. He attributes their formation to the migration of carbon from within the sheet during batch annealing of tight coils, and states that
surface carbides are extremely tenacious and can cause problems in hot dip and electrogalvanizing coating processes as well as interfering with phosphating and electrotinning processes.
Consistent with McFarland's observations, it has been found that untreated DQSK steels often develop surface carbides that cover 5 percent or more of their surface area when box annealed above the A.sub.1 temperature. Further, it has been found that surface carbides cause poor adhesion of coatings, for example, zinc-iron (Zn-Fe) electrocoatings.
Some moderate success in minimizing surface carbide formation has been achieved by avoiding excessive times at high temperatures during the annealing process. This approach, however, has not proved to be commercially viable. Process control is difficult, productivity decreased, and manufacturing costs increased.
Phosphorus-containing coatings have been used for various purposes in steel-related applications. U.S. Pat. No. 3,308,042 discloses a process for producing electrolytic tin plate having superior corrosion resistance from steel strip treated with phosphate solutions prior to annealing. Phosphorous-containing solutions having pH between 2 and 7 and between 500 and 1500 ppm phosphate ion (0.05-0.2% phosphorous by weight) are disclosed. The treated strips are provided with surface films of phosphate ion of 0.15-0.35 mg PO.sub.4 /ft.sup.2, equivalent to 0.05-0.11 mg P/ft.sup.2. Essentially the same pre-anneal phosphate treatment is disclosed in U.S. Pat. No. 3,382,110, entitled Treatment of Ferrous Metal.
U.S. Pat. No. 2,501,846 discloses a process for producing silicon steel sheet stock having a high surface resistivity. In this process, silicon steel sheet or strip stock is passed through a phosphoric acid solution and then heat treated in a continuous furnace. This initial heat treatment may be followed by a box anneal, if desired. The process employs solutions ranging from 7.25% to 50% phosphoric acid by weight, equivalent to 73 to 646 grams PO.sub.4 per liter of solution. The process provides, on average, a coating of 0.006 ounce P/ft.sup.2, or 170 mg P/ft.sup.2.
U.S. Pat. No. 2,748,037 discloses a method of treating stainless steels which are subsequently annealed, cooled in air, pickled to remove scale, and washed. The treatment consists of coating the steel with solutions which may contain, among other things. trisodium phosphate and phosphoric acid. The purpose of the treatment is to render foreign materials, e.g., dust and dirt, on the surface of the steel more readily removable during the pickling step, resulting in a smooth, uniform surface texture.
U.S. Pat. No. 3,104,993 discloses a process for galvanizing sheet metal on one side only. This is accomplished by providing on one side of the sheet metal a zinc barrier coating. The coating is applied as an aqueous colloidal solution of a refractory metal oxide and phosphoric or chromic acid, which is then heat treated before the sheet metal passes into the galvanizing bath.